You have to reinstall the game. Unfortunately, the only two EPs that are formatted to run with Lion are Seasons and Bon Voyage, therefore some trickery upon install is required. After you install the basegame, patch it with this: http://www.macgamefiles.com/item/18775/ (launch it to make sure it works) and then continue installing the other EPs. You can not install them with the installer they come packaged with. Instead, follow these instructions:
Double-click the University disc like normal, but then control click on the installer. Now go into contents/resources and control click on University.pkg and show contents. Now go to contents and drag Archive.pax.gz to your desktop. Unpack it, and drag the files The Sims 2, Package Installer, Body Shop, and Expansion Packs into your game folder.
Make sure you do not overwrite the EP folder everytime you install a new EP! Open each individual 'Expansion Packs' folder after University and drag and drop the unique folder (ex. Nightlife) into the 'Expansion Packs' folder you dragged and dropped from University. You will have to patch each EP and then launch them in between installs (I don't know why, but that's the only way I got it to work).
Good luck!